Classic voltage dividers: resistive, inductive or capacitive, have a number of drawbacks that will be discussed further below.
Firstly, the voltage divider ratio varies in time due to components ageing or ambient conditions such as temperature, humidity or environment pollution. For instance, changes in air humidity cause changes in the leakage resistances shunting the divider impedances and thereby result in uncontrolled changes in the divider ratio. Similarly, external pollutants (dust, fat) shunting the divider elements give rise to uncontrolled changes in the divider ratio. Moreover, loading a divider with finite impedance also changes the divider ratio.
Also dynamic properties of a voltage divider are subject to changes. Besides of resistance components, each voltage divider comprises parasitic reactance elements. Inductive dividers inherently comprise reactance elements. Capacities of parasitic capacitances are varying in an uncontrolled manner due to the presence or relocation of extraneous conductors in the voltage divider vicinity. Changes in the voltage divider geometry due to thermal expansion of its elements result in changes of capacities present in the divider. Uncontrolled changes in capacity can be caused by temperature and humidity or even by air pressure influence on the air dielectric constant. Furthermore, ageing of dielectric material also results in uncontrolled changes in capacitance. Since each conducting element has an inductance, an undetermined parasitic magnetic coupling always occurs between the main circuit and conducting elements in its vicinity. Parasitic capacitances or inductances form resonant circuits that substantially change the voltage divider metrological properties, particularly for signals containing high harmonics. There are known instances of ferroresonance occurrences in inductive voltage dividers that substantially distort the amplitude and frequency characteristics of a voltage divider.
It was therefore purposeful to develop an adaptive voltage divider avoiding at least a part of these disadvantages.
From the United States patent specification No. U.S. Pat. No. 3,256,484 is known a high voltage oscilloscope test probe whereof high quality of insulation is provided by a dielectric fluid that partially fills a container in which a voltage divider is enclosed. Since the employed freon dielectric fluid boiling point is +4.1 degrees Celsius the remaining part of the container is filled with the dielectric fluid vapour under pressure. Thus the insulation strength in the voltage divider surrounding is increased several times with respect to that of air. The probe allows measuring high voltages up to 30 kV within frequency range DC-70 MHz and a rise time of approximately 3.3 nanoseconds.
From the Russian patent application No. RU 2399920 is known a method for contactless measuring of voltage and current distortion factor in electric traction lines in which electric field and magnetic field sensors are fixed at the end of a two-meter rod and placed below a trolley wire. The signals of both sensors are applied to inputs of a dual channel oscilloscope connected with a computer by means of which a synchronous recording and spectral analysis of signals are performed.
A disadvantage of the probe according to U.S. Pat. No. 3,256,484 is the necessity for maintaining a specified amount of freon in the probe. On the other hand, polymer insulating materials, due to unavoidable surface currents do not ensure stable capacitance and conductance parameters under varying ambient conditions (humidity, dust). This results in an uncontrolled change in the probe both static and frequency metrological properties.